This disclosure relates generally to motor control. More particularly, this disclosure relates to a method of estimating rotor position information for a motor.
Electric motor-generators and wound field synchronous machines (“WFSM”) are types of electric machines that are controlled to provide a desired performance. A WFSM is particularly useful for aeronautical applications because the WFSM can selectively operate as a starter (motor) for a turbomachine, and as a generator that is driven by the turbomachine.
The WFSM is typically coupled to a permanent magnet generator. The permanent magnet generator powers an excitation system that provides current to windings of the WFSM when the WFSM is operating in a generate mode. Position information regarding motor components is necessary to achieve accurate control of the WFSM. Various position determination techniques have been proposed.
One known technique of estimating the position of components in the WFSM involves superimposing a carrier voltage signal upon a fundamental control voltage signal. A controller generates the fundamental control signal, which modulates an AC power source that drives the motor to produce rotational torque. As the carrier voltage signal is a relatively high-frequency signal, the carrier voltage signal does not substantially affect the fundamental control signal driving the motor. The technique of estimating the angular position of the rotor is often referred to as the carrier injection sensorless (“CIS”) method and is described in U.S. Pat. No. 5,585,709.
The CIS method has proven useful but it has shortcomings. For example, the CIS method may undesirably place an increased current carrying burden on some components. Thus, other techniques have been developed, such as a technique that measures and utilizes current harmonics of an exciter rather than the current harmonics induced by a carrier voltage signal. An example of this technique is described in U.S. Pat. No. 7,132,816. This technique also has shortcomings. For example, this technique is typically useful only for motors having certain numbers of poles and configurations of poles.
There is a need for an improved technique that takes advantage of known control strategies yet provides more reliable position information.